Default avatar generation

ftw.avatar generates default avatars for users which have not yet defined
an avatar (Personal portrait) in the personal preferences.
The avatar is generated with a random background color and two characters of
their name.

Examples:

Plone patches

Maximum user portait scale

Products.PlonePAS limits the maximum size of user portraits
(a.k.a. avatars) to 75x100.
ftw.avatar increases the size to a maximum of 300x300, allowing to
build pages with bigger avatars such as user pages.

Default user portrait patch

ftw.avatar patches Products.PlonePAS to generate a default avatar
when the user portrait is retrieved the first time and the user has not yet
set an avatar.
membership_tool.getPersonalPortrait() is patched.

Avatar scaling

ftw.avatar extends the avatar default view to accept a scaling parameter.
The size parameter is used as maximum width and maximum height for
the scale.